The present invention relates to a thermal photoelectric switch circuit. More particularly, the present invention relates to a low cost photocontrol switching circuit that is adaptable for use in multiple voltage applications.
A thermal photoelectric switch circuit (also referred to as a "photo control") is used to control outdoor lighting, such as street lights, security lights, and sign lighting. A thermal photo control switch circuit includes a heating element controlled by a photocell. During daylight, heat dissipated by the heating element induces movement in a thermally-dynamic actuator to operate a load switch.
An example of a conventional photocontrol circuit is illustrated in FIG. 1. A power source is connected to a line input 12 and a load is connected to an output 14. A photocell 16 is connected in a first current path 18 between the line input 12 and a neutral 18. The photocell 16 is responsive to ambient light so that it is conductive in the presence of light, e.g. daytime. Also in the first current path 18 is a heating element 22 which may be a resistor. A bimetal element 24 is located in proximity to the heating resistor 22. A switch 26 is located in a second current path 28 between the input 12 and the output 14. The switch 26 is connected to the bimetal element 24 and operated by it. During daylight, the photocell 16 has a low resistance thereby allowing current through the first current path 18. The flow of current in the first path 18 causes the heating resistor 22 to generate heat. This in turn causes the bimetal element 24 to move the switch 26 into an open position so that the load connected to the output 14 does not receive power from the source connected to the input 12. In the presence of darkness, the photocell 16 is nonconductive. This prevents the flow of current through the heating resistor 22 which in turn causes the bimetal element 24 to move the switch 26 to a closed position to connect the source to the load.
From time to time, circuits of this type need to be replaced. A problem that is encountered is that different municipal systems use different source voltages. Accordingly, there is a need for a low cost photo control that can be installed in various municipal systems, and preferably without the need for the installer to know the circuit supply voltage.
There are two known types of photocontrols directed to this purpose. A first type of photocontrols is a wide range type that will operate continuously over the entire expected voltage range (120 through 277 volts). This wide range type typically requires relatively robust components to tolerate the increased heat resulting from high voltage operation, while still having adequate sensitivity to operate at low voltage. This adds substantially to the product cost.
A second known type of photocontrol circuit is an adaptive type. An example of this known type of circuit 30 is shown in FIG. 2 wherein like components are indicated by the same numerals as in FIG. 1. This second known type of photo control circuit permanently changes its operating characteristics once connected to a high line voltage (208-277 volts), This type of circuit may be a dual-range device (120 nominal volts or 208 through 277 nominal volts) where the high voltage range operation is allowed by a series resistor 34 that limits the current in the first current path 18 when high voltage is applied. This series resistor 34 is shunted (bypassed) by a fuse 36 located in a current path 38 that bypasses the series resistor 34. The fuse 36 is of an appropriate current rating, so that, with low voltage (120 volts) applied, the current through the fuse 36 is not high enough to blow the fuse, thus the fuse remains intact and shunts the series resistor 34. In this condition, the photo control circuit 30 operates as if it were a standard single-voltage 120 volt photocontrol as in FIG. 1. However, the first time that this photocontrol circuit 30 is connected to a high voltage (208 through 277 volts), the associated higher current causes the fuse 36 to blow, removing the shunt 38 around the series resistor 34. With the fuse 36 blown, the photocontrol 30 is appropriately configured to operate in the high voltage range. The photocontrol circuit 30 of FIG. 2 is less costly than a wide range design, at the expense, however, of not being reversible, i.e. once the fuse 36 has blown, the photocontrol circuit 30 may only be used in the high voltage range, due to the series resistor. This is seldom a drawback in typical applications.
Although the adaptive type of photocontrol circuit offers a cost advantage over the wide range type, the adaptive type of photocontrol circuit also has certain disadvantages. It has been found that commercially available adaptive type photocontrols have relatively poor reliability. Conventional fuse shunts frequently blow unintentionally with only 120 volts applied due to normal power line surges. Also, the fuse shunts frequently fail to blow when required, particularly at the low tolerance end of the high voltage range, i.e., 208-15% (or 187 through 208 volts). When the fuse shunt fails to blow when required, the mating components are eventually damaged by overheating. Thus, commercially available photo controls that use such fuse shunts often do not satisfy the combined requirements of a very tight fusing current tolerance as well as a high resistance to nuisance blowing due to transient voltage surges or physical shock.
Accordingly, it is an object of the present invention to provide an improved photocontrol circuit that can be used with a wide range of source voltages and that overcomes the deficiencies of the prior art.